[Ada] Fix Default_Storage_Pool aspect handling in generic instantiations

[Ada] Fix Default_Storage_Pool aspect handling in generic instantiations

2018-07-16  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch12.adb (Analyze_Package_Instantiation): Handle properly an
	instance that carries an aspect Default_Storage_Pool that overrides a
	default storage pool that applies to the generic unit. The aspect in
	the generic unit was removed before copying it in the instance, rather
	than removing it from the copy of the aspects that are appended to the
	aspects in the instance.

From-SVN: r262724